This invention relates to the removal of sulfur from sulfur-containing fluid streams such as cracked-gasolines and diesel fuels. In another aspect, this invention relates to the reactivation of a sorbent composition which has become deactivated by sulfation.
Hydrocarbon-containing fluids such as gasoline and diesel fuels typically contain a quantity of sulfur. High levels of sulfur in such automotive fuels is undesirable because oxides of sulfur present in automotive exhaust may irreversibly poison noble metal catalysts employed in automobile catalytic converters. Emissions from such poisoned catalytic converters may contain high levels of non-combusted hydrocarbons, oxides of nitrogen, and/or carbon monoxide, which, when catalyzed by sunlight, form ground level ozone, more commonly referred to as smog.
Much of the sulfur present in the final blend of most gasolines originates from a gasoline blending component commonly known as xe2x80x9ccracked-gasoline.xe2x80x9d Thus, reduction of sulfur levels in cracked-gasoline will inherently serve to reduce sulfur levels in most gasolines, such as, automobile gasolines, racing gasolines, aviation gasolines, boat gasolines, and the like.
Many conventional processes exist for removing sulfur from cracked-gasoline. However, most conventional sulfur removal processes, such as hydrodesulfurization, tend to saturate olefins and aromatics in the cracked-gasoline and thereby reduced its octane number (both research and motor octane number). Thus, there is a need for a process wherein desulfurization of cracked-gasoline is achieved while the octane number is maintained.
In addition to the need for removing sulfur from cracked-gasoline, there is also a need to reduce sulfur content in diesel fuel. In removing sulfur from diesel fuel by conventional hydrodesulfurization, the cetane is improved but there is a large cost in hydrogen consumption. Such hydrogen is consumed by both hydrodesulfurization and aromatic hydrogenation reactions. Thus, there is a need for a process wherein desulfurization of diesel fuel is achieved without significant consumption of hydrogen so as to provide a more economical desulfurization process.
To satisfy such needs, there has been developed a process for desulfurizing cracked-gasolines or diesel fuels wherein a sorbent comprising zinc oxide is contacted with the sulfur-containing fluid stream under conditions sufficient to remove at least a portion of the sulfur from the fluid stream and provide a sulfurized sorbent comprising zinc sulfide. The sulfurized sorbent is thereafter contacted with an oxygen-containing regeneration stream under conditions sufficient to convert at least a portion of the zinc sulfide to zinc oxide, thereby providing a regenerated sorbent. The regenerated sorbent can then be contacted with a reducing stream to provide an activated sorbent. Thereafter, the activated sorbent can, once again, be contacted with the sulfur-containing fluid stream.
During such a hydrocarbon desulfurization process, wherein a zinc oxide-containing sorbent is employed, certain conditions can cause sulfates to form on/in the sorbent composition. Such sulfation of the sorbent is undesirable for a number of reasons. For example, the presence of excess sulfates on/in the sorbent reduces the sulfur-loading capabilities of the sorbent and thereby effectively deactivates the sorbent. Further, once an excessive amount of sulfates has formed on/in the sorbent, it can be difficult to remove the sulfates from the sorbent under conventional regeneration and activation conditions.
Accordingly, it is an object of the present invention to provide an improved process for activating a sorbent composition which has become deactivated due to sulfation.
Another object of the present invention is to provide a desulfurization process which extends the useful life of a sorbent composition by employing a unique process for removing sulfates from the sorbent.
It should be noted that the above-listed objects need not all be accomplished by the invention claimed herein and other objects and advantages of the invention will be apparent from the following description of the invention and the appended claims.
In one aspect of the invention, a sorbent activation process is provided. The activation process comprises contacting a deactivated sorbent comprising a promoter metal and at least about 2 weight percent sulfur as sulfates with a hydrogen-containing stream in an activation zone under activation conditions sufficient to reduce the valence of the promoter metal and reduce the amount of sulfates associated with the deactivated sorbent.
In accordance with another aspect of the present invention, there is provided a sorbent activation process comprising the steps of: (a) contacting a sulfurized sorbent comprising zinc sulfide and sulfates with an oxygen-containing stream in a regeneration zone under regeneration conditions sufficient to convert at least a portion of the zinc sulfide to zinc oxide, thereby providing an at least partially regenerated sorbent; (b) contacting at least a portion of the at least partially regenerated sorbent with a reducing stream in an activation zone under activation conditions sufficient to reduce the amount of sulfates associated with the at least partially regenerated sorbent, thereby providing an at least partially activated sorbent; (c) returning at least a portion of the at least partially activated sorbent to the regeneration zone for contacting with the oxygen-containing stream, thereby providing a further regenerated sorbent; and (d) returning at least a portion of the further regenerated sorbent to the activation zone for contacting with the reducing stream, thereby providing a further activated sorbent.
In accordance with a further aspect of the present invention, there is provided a sorbent activation process comprising contacting a deactivated sorbent comprising a promoter metal and at least about 2 weight percent sulfur as sulfates with a hydrogen-containing stream in an activation zone under activation conditions sufficient to reduce the valence of the promoter metal and reduce the amount of sulfates associated with the deactivated sorbent, thereby providing an activated sorbent. The activation conditions include an extended activation residence time of more than about 1 hour.
In accordance with a still further aspect of the present invention, there is provided a desulfurization process comprising the steps of: (a) deactivating a sorbent comprising a promoter metal and zinc oxide, thereby providing a deactivated sorbent comprising sulfates; (b) contacting at least a portion of the deactivated sorbent with a hydrogen-containing stream in an activation zone under activation conditions sufficient to reduce the amount of sulfates associated with the deactivated sorbent, thereby providing a reactivated sorbent; and (c) contacting at least a portion of the reactivated sorbent with a sulfur-containing fluid in a reaction zone under reaction conditions sufficient to remove at least a portion of the sulfur from the sulfur-containing fluid, thereby providing a desulfurized fluid and a sulfurized sorbent.